The present invention relates generally to ignition systems for internal combustion engines and particularly to breakerless magneto-type ignition systems. The present invention provides an improved trigger circuit for inducing a sparking potential in a secondary winding of an ignition coil by abruptly changing the rate of current flow through the primary winding of the ignition coil.
In one prior art magneto-type ignition system, the trigger circuit includes a generating coil for supplying current flow to the primary winding of an ignition coil and a transistor connected in parallel with the primary winding to control the current flow through the primary winding. When rendered conductive, the current from the generating coil flows through the transistor, which acts as a short circuit across the primary winding. Subsequently, the current flow through the transistor is interrupted by turning off the transistor (rendering it non-conductive). At this time, the voltage generated is impressed upon the primary winding, causing an abrupt change in current flow through the primary winding and thereby inducing a sparking potential in the secondary winding. However, in this ignition system the power required to control the switching of the transistor, that is the power required to drive the base of the transistor, is provided by the generating coil. For example, the output voltage of the generating coil is divided by connecting the series circuit of a plurality of resistors in parallel with the generating coil so that the divided voltage is guided to the base of that transistor. Since the current output of the generating coil partially flows through those resistors, the current through the primary winding is reduced. As a result, the changing rate of current flow through the primary winding due to the interruption of the current flow through the transistor is also reduced, thereby decreasing the voltage induced in the secondary winding. It will be appreciated that the effect of this undesirable reduction in ignition performance is especially pronounced when the internal combustion engine is running at slow speeds, as the magnitude of the voltage output from the generating coil is lower than at high operating speeds.
Accordingly, it is a principle object of the present invention to provide a trigger circuit for a magneto-type ignition system, in which the magnitude of the voltage or sparking potential induced by the secondary winding of the ignition coil is increased by increasing the changing rate of the current flow through the primary winding, thereby improving the performance of the ignition system especially at slow engine speeds.
It is another object of the present invention to provide a trigger circuit which automatically advances the ignition timing with an increase in engine speed.
In accordance with the foregoing objects, the present invention provides a novel trigger circuit which features two generating coils, two semiconductor switching elements and a capacitor. The first generating coil is used to supply current flow to the primary winding of the ignition coil, and the first semiconductor switching element (a transistor) is connected across the primary winding in parallel to control current flow through the priimary winding. The second semiconductor switching element (a thyristor) is used to render the first semiconductor switching element non-conductive in cooperation with the capacitor. An important feature of the present invention is the second generating coil, which is used to supply the power required to charge the capacitor and to control the conduction of the first and second semiconductor switching elements. During one half wave voltage output of the second generating coil, the capacitor is charged and the first semiconductor switch is rendered conductive. During the other half wave of the voltage output from the second generating coil the second semiconductor switching element is rendered conductive. The conduction of the second semiconductor switching element is operative to discharge the capacitor and render non-conductive the first semiconductor switching element. This causes an interruption of the current flow through the first semiconductor switching element, and the current flow from the first generating coil is directed through the primary winding of the ignition coil to induce a sparking potential in the secondary winding of the ignition coil. Thus, it should be appreciated that since the first generating coil is not used to control any switching elements, all or substantially all of the current flow from this coil is directed through the primary winding, thereby increasing the changing rate of current flow through the primary winding.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: